This invention relates to the designs of spatially resolved spin resonance detection, including but not limited to electron spin resonance (ESR) and nuclear spin resonance (NMR), with high sensitivity.
For many applications in materials and bioscience research, spatially resolved spin resonance detection with high sensitivity is desired. Conventional spin resonance detection experiments are usually performed by placing a sample in a microwave cavity or a pair of RF coils situated in a strong DC or substantially static magnetic field that is perpendicular to the microwave or RF magnetic field. High power microwave or RF radiation excites the coherent spin precession. Precessing spin-induced induction and absorption signals are picked up by cavity or coil and detected by diode mixer. Although the intrinsic sensitivity is limited by cavity Johnson noise, which is near single-spin detectivity, this level of detection has never been possible practically. Primary limitations in a conventional experiment are large background noise from high power excitation signal generated by high-power klystron source (need to excite spin in bulk samples) and diode detector noise since low noise amplifier cannot be employed before diode detector without being saturated by high level excitation signal pick up at detection port.
What is needed is an approach that provides spin resonance detection, preferably spatially resolved to within 0.5 xcexcm to 1 mm. Preferably, the approach should avoid detection of background signals, such as the strong input or excitation signal, and should not require use signal levels that are at or above a saturation threshold.
These needs are met by the invention, which provides methods and systems that allow evanescent microwave or RF wave excitation and detection of spin resonance and cancellation of excitation signal at the detection port, and therefore allow spatially resolved (with spatially resolution better than the wavelength of the excitation signal) spin resonance detection with high sensitivity limited by the low noise amplifier or Johnson noise of the resonator.
In a first embodiment, an evanescent microwave orthogonal resonator probe is designed to achieve high spatial resolution and high sensitivity spin resonance detection. The probe design uses a bimodal transmission-type (or waveguide/cavity type) resonator with input and output coupled to orthogonal modes. The purpose of the design is to isolate excitation and detection modes and dramatically reduce background signal. This will allow low noise amplification to be implemented to achieve Johnson noise limited sensitivity without amplifier saturation. Only a very small spin resonance induced signal power is coupled to the probe output and amplified. Spin resonance signal is proportional to excitation RF or microwave field intensity (Hrf)2 below saturation threshold. In conventional spin resonance studies, very high power (klystron source) is required to reach the saturation level. In this design, only very small microwave power is needed to excite small volume of sample near the tip (or aperture) with very high field intensity. This will dramatically reduce the background noise of microwave source because a low noise level generator can be used. Because both excitation and pickup volume are small, other extrinsic noises will also be dramatically reduced. A typical klystron source has FM noise around -30dB while a high quality microwave synthesizer proposed to implement in this project has FM noise around -110dB.
In a second embodiment, internal or external cancellation schemes are used to cancel the large excitation signal that would otherwise be detected at the detection port. In an external cancellation scheme, two substantially identical transmission line resonators (or cavity/waveguide resonators) with the same resonant frequency and insertion loss are used. One of the resonators has an evanescent wave probe to interact with sample (excite and detect spin resonance). The evanescent probe may be (1) a metal tip connected to the center conductor of a transmission type resonator, (2) a metal loop connected between the center conductor and ground shielding; or (3) an aperture on the shielding wall of a cavity or waveguide resonator. The other resonator serves as a reference or xe2x80x9cdummyxe2x80x9d resonator. The outputs of the two resonators are received by a power combiner with a 180xc2x0 phase-shift for one of the input signals, thereby forming a difference of the output signals. This signal combiner cancels the large background (excitation) signal, and a low noise amplifier can be employed to increase the sensitivity. As the evanescent probe excites the spin resonance, the probe-sample interaction breaks the symmetry and precise cancellation, and the resulting small changes represent the spin resonance signal to be detected.
In the case of internal cancellation scheme, the pick up coupling will be positioned at the node of the mode of either a transmission type or cavity type resonator, equipped with evanescent probe as described above. As the evanescent probe excites the spin resonance, the induced magnetization will change the node position in the resonator. As a consequence, the pick up coupling will detect the small signal due to this spin resonance induced effect without coupling to the large excitation background signal. Low noise amplifier can then boost the signal to achieve high sensitivity in both cases.
In each of these situations, in order to further increase the sample volume sensitivity, (e.g., increase the spin states population difference, especially for NMR at room temperature), optical pumping is preferably used. An integrated optical path is designed to apply optical pumping.
In a third embodiment, an optical pumping and detection scheme is employed, using an evanescent electromagnetic wave excitation. The integrated evanescent microwave probe-optical microscope system (EMP-OM) can provide microwave induced optical detection for optically pumped spin resonance detection. Spatial resolution in both operating modes is achieved by an evanescent microwave (or radio wave) probe, which only excites and picks up spin resonance induced microwave (or RF) induction and absorption signals in a very small sample volume (as small as nm3) proportional to the cube of probe radius.